1. Open-Cut Excavation (Mass Excavation)
Description:
Open-cut excavation involves digging without internal support or bracing structures, allowing for large-scale excavation equipment and efficient construction of the main structure. This method is fast and cost-effective, suitable for sites with low groundwater levels and ample space.
Advantages:
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No retaining or bracing structures required (except for high-permeability strata, where water cutoff structures are necessary).
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Easy excavation and quick soil removal.
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Convenient for internal civil structure construction.
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Lower construction costs.
Disadvantages:
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Higher soil removal volume; backfilling may increase costs if no stockpile area is available.
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Limited excavation depth (generally ≤10m).
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Requires low groundwater levels or dewatering measures if no water cutoff structure is used.
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Needs open space with no nearby protected structures.
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Slope protection measures may increase costs during rainy seasons or long construction periods.
2. Gravity Cement-Soil Wall Method (Shallow Trench Support)
Description:
This method is environmentally friendly, producing no pollution, noise, or vibration. It is cost-effective and allows for unobstructed excavation and construction inside the pit.
Advantages:
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No internal bracing required.
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Lower construction costs.
Disadvantages:
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Low strength of cement-soil mixing piles (typically 0.8 MPa).
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Significant environmental impact during construction.
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Requires trapezoidal overlapping construction.
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High cement consumption (13–15%).
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The large construction site is needed.
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Significant wall deformation.
3. High-Pressure Jet Grouting Pile Method
Description:
This technique forms piles of various diameters (single, double, or triple-fluid systems) and can bypass underground utilities. It serves as a water cutoff curtain and can create self-supporting walls, eliminating the need for internal bracing.
Applications:
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Suitable for soft soils, clay, silt, sand, loess, and gravel.
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Not ideal for soils with large boulders, hard clay, or organic content.
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Used in foundation reinforcement, deep excavation waterproofing, slope stability, and underground pipeline reinforcement.
Advantages:
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Functions as a gravity retaining wall or water cutoff structure.
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Higher strength than cement-soil mixing piles (1.2–2.0 MPa).
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Less environmental impact.
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Flexible obstacle avoidance.
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Larger diameters (0.6–2.5m).
Disadvantages:
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Higher cost than cement-soil mixing piles.
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High cement consumption (20%).
4. Soil Nailing Wall Method
Description:
Best for low-water-table cohesive soils, this method uses soil-wall interaction for stability, offering fast construction and minimal environmental impact.
Applications:
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Temporary/permanent retaining structures.
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Slope stabilization, foundation underpinning.
Advantages:
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Fast construction, no internal bracing.
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Lower deformation than gravity walls.
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Cost-effective.
Disadvantages:
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Requires layered excavation.
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Waterproof curtains are needed in high-water-table soft soils.
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Difficult quality control; low nail load capacity.
5. Caisson Method
Description:
Minimal footprint, no additional retaining structures, and low impact on surroundings.
Types:
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Wet Caisson: Uses slurry or compressed air.
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Dry Caisson: Workers excavate inside.
Advantages:
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Cost-effective, structurally robust.
Disadvantages:
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Difficult to correct large caisson deviations.
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Challenging soil removal in wet conditions.
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Significant ground settlement.
6. Deep Excavation Methods
6.1 Diaphragm Walls
Types:
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By construction: Pile, panel, composite.
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By material: Reinforced concrete, plastic concrete, steel.
Applications:
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Depths >10m.
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Permanent structural walls with high waterproofing needs.
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Top-down construction.
6.2 Sheet Piles
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Used for shallow slopes/excavations.
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Low noise/vibration alternatives are available.
6.3 Soldier Pile Walls
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Composed of drilled/bored piles.
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Requires capping beams for stability.
6.4 External Anchoring (Ground Anchors)
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Installed in pre-drilled holes for tension resistance.
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Ideal for unobstructed excavation.
Conclusion
Choosing the right underground construction method depends on soil conditions, depth, groundwater, and project requirements. Each technique offers unique benefits, from cost-effective open-cut excavation to robust diaphragm walls for deep projects. Proper selection ensures safety, efficiency, and cost control.
